IPTV: Revolutionizing the TV industry
Jun 1, 2007 12:00 PM, BY TOM NEWBERRY AND JOSEPH WEBER
Multimedia over IP
Multimedia and networking is core to IPTV. Multimedia applications use various media types, such as text, graphics, animations, audio and video. There are many network-based multimedia applications today. Furthermore, there are many bright and imaginative minds working on ideas for applications intended for high-speed bidirectional networks.
Figure 3. Broadcast video using multicast in a broadband network
Click image to enlarge.
Networked multimedia applications are important, so it is critical for the IPTV network architect or content creator to understand the issues associated with multimedia networking as well as understand what tools can enable effective and compelling new applications.
Within the network, multimedia data can be affected in the following ways: dropped packets, jitter between packet delivery times, delayed packets and data corruption. Even when the TCP protocol is used, the effectiveness of the IPTV service can be affected by the reliability and speed of the network.
The goal of quality of service (QoS) is to make sure the network can deliver end-to-end data with expected and predicted results. This includes latency, error rates, up-time, bandwidth and network traffic loads.
QoS can be extremely important to a successful IPTV service within a congested network. Only service operators that also own and manage the IP network to homes can guarantee QoS for the service. IPTV services that use the general Internet are not guaranteed the QoS necessary for a good user experience.
Real-time transport protocol
IP networks were not designed for real-time delivery of data and can have unpredictable jitter and delay. The multimedia data that travels on the IP networks must arrive on time and in the same order it was sent. Real-time Transport Protocol (RTP) addresses the time-critical requirement of multimedia bit streams. It provides a timestamp and sequence number to IP packets containing media data. These can be used by the receiving device to synchronize playback and manage buffers for network jitter.
Encapsulating media data into IP packets
Delivery of media bit streams over IP requires several layers of encapsulation. MPEG-2 transport streams, for example, consist of a series of 188-byte packets. These are grouped together and wrapped within an RTP packet. Finally, the RTP packet is encapsulated within a TCP or UDP datagram, forming an IP packet.
Figure 4. An RTP packet encapsulated in UDP/IP
Click image to enlarge.
Figure 4 shows an RTP packet containing several MPEG-2 transport packets within its payload, all encapsulated using UDP in an IP packet. This diagram shows seven 188-byte transport packets that constitute the RTP payload.
Each encapsulation adds additional header data and therefore reduces the bandwidth efficiency. If the network has sufficient QoS, it is possible to deliver media packets without the overhead of RTP. The packets are instead inserted directly into UDP packets. Figure 5 shows how MPEG-2 transport stream packets can be encapsulated within a UDP/IP packet.
Sending MPEG-2 transport stream packets over UDP is used extensively within the private networks of cable and telephone companies to deliver MPEG-2 transport streams throughout the system. For general delivery over the unmanaged Internet without QoS guarantees, streaming protocols such as RTP need to be used, but even then, packets may be lost in delivery, resulting in artifacts in the media presentation.
Figure 5. MPEG-2 transport stream encapsulated in UDP/IP
There are other system layer standards besides MPEG-2 and next-generation compression methods such as AVC H.264. While the encapsulation methods may differ slightly, they all require either strong QoS or overhead to ensure timely delivery of media packets.
Summary
Delivering TV and movie services over IP promises to revolutionize almost every component of the television industry. Just as the Internet changed the way we shop, read the news and personally interact, television services over IP could change how we integrate television entertainment into our daily lives. Decades old business models may change as a result of this technological shift to IPTV.
The Internet and the protocols used within it were not designed for the real-time delivery of multimedia content. Assuring the required QoS over a network may require additional protocols and additional bandwidth to overcome the inherent limitations of IP.
Tom Newberry is product development manager for Thomson, and Joseph Weber, Ph. D., is director of product management for TiVo.
| Want to use this article? Click here for options! |  |
